A. Field of the Invention
The present invention relates to the provision of interconnections among digital components and, more particularly, to an improved method and means for establishing an optical interconnect network among board-mounted digital devices.
B. Description of the Related Art
A modern electronic computation module or processor typically comprises an array of integrated-circuit ("IC" or "chip") devices mounted on a planar circuit board (which may consist of more than one layer); the board also contains various other electronic components. Metal paths or tracks on the board establish electrical interconnections among the pins or leads of these devices.
Improvements in mask generation techniques have increased the number of electronic components that may be housed in a single IC, leading to so-called "very large scale" integrated circuits ("VLSICs") that can contain many thousands of such components. Naturally, VLSICs usually require more terminal pin connections than less complex devices, and incorporation of multiple VLSICs on a single board therefore increases the density of the necessary interconnection paths.
Unfortunately, limitations associated with ordinary interconnection techniques can restrict the practical number of VLSICs that may be mounted on a single board, thereby limiting the computational power of such a board. Electromagnetic interference between closely spaced connection path can distort signal transmission. Unavoidable capacitances involving these paths can also cause faulty data transfer, particularly at high data-transfer rates. To the extent that these problems can be ameliorated by careful board design, the extended design and testing process, followed by necessary quality-control measures during production, can significantly increase the average cost of highly intricate boards.
Interconnection limitations can also arise at the device level: highly complicated VLSICs may require more terminals for operation than conventional packaging designs can physically support.
Indeed, the very notion of fixed interconnection paths may impede full exploitation of the capabilities offered by complex VLSICs. For example, "parallel" computational designs that include multiple, cooperating VLSIC processors may require selective communications among the processors, thereby demanding overlapping or alternative connections that are either topologically or electrically impracticable using the conventional techniques.
To address these difficulties, various optical interconnection schemes have been proposed. These approaches involve on-chip conversion of electrical output signals into optical signals that are transmitted over some form of "optical interconnect" communication channel to connect one chip to one or more other chips capable of demodulating and decoding these signals. Currently, the most practical mode of light transmission appears to involve optical fiber conduits. These conduits are capable of transmissions at large data-transfer rates without cross-coupling or stray capacitance problems, and they thus can be spaced closely or even overlapped without regard to capacitive or magnetic interaction. Optical fibers can also be manufactured at sufficiently small diameters to significantly increase the number of terminals that may be conveniently associated with a single VLSIC.
Despite its advantages, the optical-fiber solution is far from perfect. Because they cannot be bent into small-radius curves, the optical-fiber interconnections must occupy a relatively large volume, thereby diminishing some of the benefits of device miniaturization. Furthermore, once established, the optical-fiber interconnections are fixed and permanent; consequently, like ordinary interconnections, they cannot be dynamically altered during operation of the associated components.
Another technology presently under development involves data transmission along free-space holographic light beams. Again, even if practical, such techniques would provide a set of fixed (although possibly alterable) interconnection paths, and would also require a large spatial volume due to the long focal lengths generally associated with the necessary optical elements.
Finally, researchers are currently exploring the possibility of using movable on-chip lasers, each capable of scanning through a hemisphere of space, to transmit data by bouncing beams of light off a specular mirror to strike detectors on ICs of interest. While capable of producing dynamically changing interconnects, this technique would require highly refined movement and position-sensing mechanisms, as well as specialized miniature lasers. In addition, each single on-chip laser element would establish only a fixed number of interconnections at a given time. As far as is known, no practical embodiments have as yet been demonstrated.